The present invention falls into the field of echo annexation devices which use coil springs as materials for a signal retardation device.
Generally, in the echo annexation device, the mechanical vibration impressed on a coil spring by the drive-transducer is helically transmitted along the coil spring, namely, along the helical direction, and concurrently the vibration causes the retardation in time at both ends of the coil spring by repeating intermittently the reflection. Consequently, the retardation said in time generates the echo.
The intermittently repeating reflex movement induces the inherent compressive movement of the coil spring and results in casing the stationary wave, which modulates the echo vibration modulated, and causes flutter sound on the part of the pickup-transducer. Therein the frequency characteristics are marred so much that distortion develops.
To prevent this distortion, various devices have been presented. So far, some devices have two kinds of coil springs different in helical direction, and other devices have a portion having a different coil pitch on the coil spring, but none of them has succeeded in preventing this distortion.